The Authorised Supply Capacity (ASC), also known as the Availability level, is the amount of electricity that the distribution company makes available for a business. This Authorised Supply Capacity is measured in Kilo Volt Amperes (kVa) and charged on a monthly basis. If a business consumes more electricity than its Authorised Supply Capacity, there will be an excess capacity charge. This is charged at least three times the normal kVa rate.

A site's usage of its Authorised Supply Capacity is determined every month by means of a Highest Maximum Demand (HMD). This Highest Maximum Demand is based on the type of metering available at the site. For sites where single-channel metering is in use, the kW value will be used instead of the kVa. For multi-channel metered sites, the kVa HMD is reached by applying a standard formula to the kW and leading and lagging data-flows for each half hour in the month. The formula to calculate the HMD is as follows: